In recent years, there has been a growing demand for lithium ion secondary batteries in applications such as portable information terminals, portable electronic devices, electric cars, hybrid electric cars, and further stationary electric storage systems. However, existing lithium ion secondary batteries use flammable organic solvents as liquid electrolytes, and require rigid exteriors so as to prevent the leakage of the organic solvents. Further, there are constraints on the structure of devices, such as the need for portable personal computers or the like to have a structure against the risk in the case of leakage of the liquid electrolyte.
Furthermore, the applications extend even to movable vehicles such as automobiles and airplanes, and large capacity is required in stationary lithium ion secondary batteries. Under such a situation, there is a tendency that the safety is considered to be more important than before, and the development of solid-state lithium ion secondary batteries without using toxic materials such as the organic solvents has been focused.
Further, not only high energy density, but also high-speed processing is required in smartphones which have been spread rapidly and widely in recent years. In order to meet such requirements, batteries are desired to have a voltage as high as possible. Accordingly, it is exceptionally important for secondary batteries for small devices to ensure such a voltage.
As a solid electrolyte in solid-state lithium ion secondary batteries, use of oxides, phosphate compounds, organic polymers, sulfides, and the like, has been investigated. However, oxides and phosphate compounds have low resistance to redox, and thus it is difficult for them to stably exist in lithium ion secondary batteries. Further, they also have a disadvantage that, when materials such as metal lithium, low crystalline carbon, and graphite, are used as a negative electrode, the solid electrolyte reacts with the negative electrode (Patent Literature 1).
Further, oxides and phosphate compounds have characteristics that their particles are hard. Accordingly, in order to form a solid electrolyte layer using these materials, sintering at a high temperature of 600° C. or more is generally required, which is time consuming. Furthermore, oxides and phosphate compounds, when used as a material of the solid-electrolyte layer, have a disadvantage that the interfacial resistance with the electrode active material increases. The organic polymers have a disadvantage that the lithium ion conductivity at room temperature is low, and the conductivity drastically decreases when the temperature decreases.
Meanwhile, it is known that sulfides have a high lithium ion conductivity of 1.0×10−3 S/cm or higher (Patent Literature 2) and 0.2×10−3 S/cm or higher (Patent Literature 3) at room temperature. Further, their particles are soft, which enables a solid electrolyte layer to be produced by cold pressing, and can easily make its contact interface a good state. However, in the case of using materials containing Ge or Si as a sulfide solid electrolyte material (Patent Literature 2 and Patent Literature 4), these materials have a problem of being susceptible to reduction. Further, there is also the following problem: when batteries are configured using negative-electrode active materials having an electrode potential of about 0 V (with reference to Li electrode) as typified by lithium metals or carbon active materials which are capable of ensuring high voltage in a single cell (Patent Literature 4), the reduction reaction of the sulfide solid electrolyte occurs.
In order to prevent the aforementioned problems, a method of providing a coating on the surface of the negative-electrode active material (Patent Literature 5) and a method of engineering the composition of the solid electrolyte (Patent Literatures 6 to 10), for example, have been proposed. In particular, Patent Literature 10 uses a solid electrolyte containing P2S5, but a concern for a reaction with the negative-electrode active material remains, even in the case of using such a sulfide solid electrolyte (Non Patent Literature 1). Further, the stability of the negative electrode easily changes due to a slight amount of impurities in the solid-electrolyte layer, and its control is not easy. Under such circumstances, a solid electrolyte capable of forming a good interface with an adjacent material while having high lithium ion conductivity without adversely affecting the stability of the electrode active material has been desired.
As to new lithium-ion-conducting solid electrolytes, it was reported in 2007 that the high temperature phase of LiBH4 had high lithium ion conductivity (Non Patent Literature 2), and it was reported in 2009 that a solid solution obtained by adding LiI to LiBH4 could maintain the high temperature phase at room temperature (Non Patent Literature 3 and Patent Literature 11; hereinafter, for example, an ion conductor containing a complex hydride such as LiBH4 will be referred to also as a complex hydride solid electrolyte). Configurations of batteries using such a complex hydride solid electrolyte have been studied, and it is disclosed that they exert effects particularly in the case of using metal lithium as a negative electrode (Patent Literature 12 and Patent Literature 13).
However, the solid electrolyte containing LiBH4 has a disadvantage of reducing oxides that are generally used as a positive-electrode active material such as LiCoO2. As a technique for preventing this, it was reported that charge/discharge cycles at 120° C. could be achieved by coating a 100-nm LiCoO2 layer formed by pulsed laser deposition (PLD) with about 10 nm of Li3PO4 (Non Patent Literature 4). However, this technique is not intended for bulk types, but for thin film batteries manufactured using vapor phase deposition, and therefore there are disadvantages that the capacity per cell cannot be ensured as much as in bulk types, and the productivity is also poor.
Although a method for avoiding the reduction by the complex hydride using a specific positive-electrode active material has been found, available positive-electrode active materials are exceptionally limited (such as polycyclic aromatic hydrocarbons with a polyacene skeletal structure and perovskite fluorides) (Patent Literature 12). Further, these positive-electrode active materials are not oxide positive-electrode active materials that are commonly used for commercially available lithium ion secondary batteries at present. Patent Literature 12 describes that oxide positive-electrode active materials coated with specific ion conductors or carbons are less likely to be reduced, but the data shown in its examples only indicates the reduction action during charge, and thus it does not necessarily describe the effects when charge and discharge are repeated.
Non Patent Literature 4 mentions that the reduction of LiCoO2 by LiBH4 occurs during charge, and FIG. 1 of Non Patent Literature 4 clearly shows that the battery resistance increases by repeating charge/discharge cycles. It can be said from this that there is a demand for effective means capable of not only suppressing the reduction of the positive-electrode active material due to the complex hydride in the short term, but also suppressing the increase in the battery resistance after repetition of charge and discharge.